As mitomycin derivatives relevant to the present invention where a substituent is introduced in the methyl group at the 6-position, there are known mitomycin derivatives where one hydrogen atom of the methyl group is substituted with deuterium (.sup.2 H) or tritium (.sup.3 H) (Japanese Published Unexamined Patent Application (hereafter referred as JP-A) 70490/89) and mitomycin derivatives where one hydrogen atom of the methyl group is substituted with RO or RS (wherein R represents hydrogen, a substituted or unsubstituted alkyl having 1 to 12 carbon atoms, a substituted or unsubstituted alkenyl having 2 to 12 carbon atoms, a substituted or unsubstituted aralkyl or a substituted or unsubstituted aryl) (JP-A 167282/90).